Nuclear Magnetic Resonance
(1) Predicting molecular structure :
An N.M.R. spectrum can reveal all the information needed to draw a structural formula for a molecule.
Example - the 1H-NMR spectrum of styrene oxide
The x-axis of the spectrum uses δ values for protons measured in ppm (parts per million), which indicate how different the resonances of the protons are to each other. This is called the chemical shift of the proton. The lower the δ value the more saturated the environment of the proton, e.g. CH3CH2CH2CH3protons at about δ=1-2 ppm. The higher the d value the more unsaturated the environment of the proton, e.g. C6H6 protons at about δ=7 ppm.
The δ values for different types of protons are given on the datasheet and reproduced here :
| type of proton : | chemical shift, δ |
| R-CH3 | 0.7 - 1.6 |
| R-CH2-R | 1.2 - 1.4 |
| R3CH | 1.6 - 2.0 |
 | 2.0 - 2.9 |
 | 2.3 - 2.7 |
| -O-CH3 -O-CH2-R | 3.3 - 4.3 |
| R-OH | 3.5 - 5.5 (but very variable) |
 | 6.5 - 7.0 |
 | 7.1 - 7.7 |
 | 9.5 - 10.0 |
 | 11.0 - 11.7 |
The different types of protons in a molecule all give rise to different peaks in the spectrum. For example, in ethanol (CH3CH2OH) there are three different types of protons :
Therefore, the N.M.R. spectrum shows three groups of peaks, each group caused by a different group of protons.
The area under a group of peaks is directly proportional to the number of protons resonating to cause that peak. So, the relative areas reveal the relative amounts of protons in the molecule. For example, in ethanol there are three groups of peaks with area ratios of 1:2:3; therefore, the ratio of the different protons in ethanol is 1:2:3.
Protons on adjacent carbon atoms interact with one-another. This causes what should be a single peak for each group of protons to be split into a group of peaks.
The so-called splitting patterns depend on the number of neighbouring protons and follow the pattern in the table below :
| number of neighbouring protons : | splitting pattern (relative peak heights): | ||||||
| 0 | 1 | ||||||
| 1 | 1 | 1 | |||||
| 2 | 1 | 2 | 1 | ||||
| 3 | 1 | 3 | 3 | 1 | |||
So, if a proton has no neighbours it is not split at all (it is a singlet peak); one neighbouring proton gives rise to a doublet; two neighbouring protons cause atriplet and three neighbouring protons yield a quartet.
The one exception to this rule is that labile protons (see below) do not generally interact with neighbouring protons and so do not cause splitting nor are split themselves.
Putting all this information together enables the structure of the molecule to be deduced. Even fine structural elements such as structural and E-Z isomerism can be seen.
(2) Predicting spectra :
This process is simply the reverse of the steps above. The table is consulted to find the correct δ values and the structure shows how many neighbouring protons each peak has and therefore what the splitting patterns should be.
(3) Use of D2O in N.M.R. :
Protons attached to oxygen and nitrogen atoms are easily removed and replaced by protons from other sources. This process is continual and generally goes unnoticed. These protons are called labile.
If an organic molecule containing hydroxyl(-OH), carboxylic acid (-COOH) or amine (-NH2) groups is mixed with deuterium oxide ("heavy" water, D2O), then the protons (1H) are replaced with deuterium atoms (2H). Since the deuterium atom has an even number of particles in its nucleus (a proton and a neutron) it does not show up in proton N.M.R.
So, if spectra are taken of a molecule before and after the use of D2O, a comparison of the two spectra can reveal any labile hydrogens in the molecule.
thank you for such informative explanation
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